216 



Dr. J. Jago on Visible Direction. 



[Mar. 13, 



tion upon the nerve-stern, tending to carry its distal extremity that way. 

 The " white circular rings, brighter at one margin than the other," have 

 been instanced as proclaiming that such traction cannot occur without 

 flexure between the nerve-stem and the eye-apple, which displays itself 

 at the junction o£ the optic disk with the surrounding retinal expansion. 

 In other words, under the concordant action of the orbital muscles, all the 

 movements of the globe are so equably coordinated that the nerve-stem is 

 never subjected to unwonted traction, and consequently always emerges 

 through the ocular tissues to open out into the retina as a normal to their 

 surfaces, in which case no visual parallax appears. But no sooner is there 

 lateral traction than the axis of the emergent nerve-stem, or of the optic 

 disk, deviates from the said normality ; and were that disk impressible 

 by objective light, its central point would deviate in the same direction, 

 and an equal deviation in visible direction would be associated with every 

 other point in the visual field. 



Hence we are fairly landed upon the conclusion that visible direction, 

 which has already been tracked backwards to the optic nerve, is a function 

 of its terminal direction, being identical with it at the centre of the optic 

 disk, both in the equable use of the eye and in the unequable. 



Finally, it is clear that if the eyeball be twisted round the axis of the 

 optic disk the terminal portion of the nerve will be twisted in the same 

 direction; and thus the opposite twisting of the visible field in certain ex- 

 periments related are explicable by the same hypothesis — an hypothesis 

 that accounts for all the phenomena of visible direction, whether regular 

 or irregular. 



Whenever the inverted retinal image, by means of nervous arrangement, 

 is ranverted, an erect image is seemingly projected, if not from, by means 

 of the base of the optic nerve. 



The principles here arrived at, when applied to binocular vision, lead 

 to the observation of phenomena that have not been before put on 

 record. 



"Wheatstone, in his classic paper in the Philosophical Transactions, 

 wherein he announces his discovery of the stereoscope and expounds its 

 theory, only speaks of stereoscopic vision from two perspectives, an appro- 

 priate one for each eye, when (no instrument beiug used) the optic axes meet 

 each other beyond them, or have previously intersected, so that each eye sees 

 the other's perspective — that is, in all experiments by him and other 

 subsequent writers on the subject the optic axes have always been supposed 

 to intersect or to lie in one plane. 



But as it has been demonstrated that the axes of visible direction need 

 not be coincident with the optic axes, it ought to follow that we may 

 continue to see bodies in relief from a pair of stereoscopic perspectives, 

 though these are not placed transversely to each other. 



Two perspectives of a pyramid are drawn, such as, when placed laterally 

 apart as is usual in stereoscopic slides, and looked at by concourse of the 



